1. Field of the Invention
This invention relates generally to an apparatus and method for reducing lift and drag of a soft top passenger vehicles by attaching a deflector along the roof of the vehicle at a previously determined location to change the separation point between attached and separated flow.
2. Background of the Invention
It is an ongoing goal of automobile designers and engineers to reduce the aerodynamic drag over the vehicle and lift on the rear axle. Reducing aerodynamic drag and lift has the effect of increasing fuel efficiency or increasing speed for a given horsepower, as well as improving handling characteristics. Even small improvements in fuel efficiency can reduce operating costs significantly.
Considerable effort has been initiated towards reducing aerodynamic drag and lift in vehicles having articulated bodies, that is, between truck tractor cabs and their associated trailer bodies. While the problem has been well-addressed in that context, resulting in widespread use of air deflectors mounted on tractor cabs, single body passenger vehicles present a different problem, as different fluid mechanics considerations apply. The need for reduced lift and drag exists there as well, in the form of a practical device to do so without changing the overall appearance of the vehicle.
The method and apparatus of the present invention are based on a balancing of two fluid mechanics effects that occur as air moves over a passenger vehicle. The first effect occurs as the air moves along the curved roof of the passenger vehicle. As it does so it accelerates locally where surface curvatures are positive. Acceleration is proportional to the magnitude of the curvation, i.e., the sharper the turn, the higher the acceleration. Accelerating flow causes low pressure, which in turn causes increased lift and drag. The higher the speed, the lower the pressure acting on the surface, and the more the drag and the lift. In most passenger vehicles, surfaces transition from being mostly upward facing to being mostly rearward facing. In the region where the transition occurs, the curvature in the surface accelerates the flow locally if the flow remains attached. The pressure in this area will be low, and since the surface in this region faces both up and back, the pressure acts to increase both the lift and drag of the vehicle.
The second effect occurs after the air stream passes the transition point. Globally, the flow decelerates in this region because the area available is increasing. It continues to do so until it separates from the surface. Beyond this separation point, a low pressure stall area exists, which again subjects the rearward facing area behind the separation point to low pressures, which again is a major contributor to drag and lift. The further forward the separation point is, the larger the area exposed to this low pressure region.
Generally, a long, gently sloped surface (e.g., the top of an airfoil) that provides a gradual deceleration of the flow and avoids separation as long as possible is favorable for decreasing the drag. However, in passenger vehicles, the curvature in the transition area from upward to downward rearward is to a degree such that the low pressure generated locally (the first effect) causes an increase in lift and drag that is larger than the overall lift and drag that would be experienced if the flow had separated earlier, despite the second effect. The purpose of the present invention is to force the flow to separate sooner, before the adverse effects of flow over the curvature (the first effect) outweigh the lift and drag penalty of moving the separation point forward (the second effect). By doing so, overall lift and drag are decreased.